Linear actuators comprising piston means displaced by fluid or air pressure are typically used to control the opening and closing of valves, e.g., gate valves and sluice gates. One of the principal problems encountered in providing actuators of this type to control valves results from the fact that the force required to break the seal upon opening the valve is generally considerably greater than that necessary at any other portion of the opening or closing cycle. This relatively large force is commonly referred to as the "break-away" force and is caused by friction of the valve disk against the valve seat on opening.
Another problem results from the fact that when the final closing force is too large, the valve may slam shut causing distortion of the parts and damage to the seals.
Various actuators have been designed with the purpose of overcoming the "break-away" force by supplying an initial larger force. U.S. Pat. Nos. 2,896,413 (Hussey), which issued Jul. 28, 1959, and 3,208,354 (Topinka), which issued Sep. 28, 1965, both disclose fluid actuators having means for providing a large initial force capable of overcoming the friction forces involved in breaking a valve seal or moving an object. Furthermore, both the Hussey and Topinka patents use auxiliary pistons to generate the large initial force.
The Hussey patent provides a working piston which provides the principal opening and closing force and an auxiliary piston which provides a supplemental force during the initial part of the opening cycle and which also acts as a buffer during the final portion of the closing cycle. An operating fluid enters an annular chamber and flows into a compartment adjacent to the primary piston. The fluid adjacent to the primary piston then flows from that compartment into another compartment adjacent the auxiliary piston by means of a conduit within the auxiliary piston. The force of the fluid pressure in the compartments acting against the pistons urges them both to the right.
The Topinka patent discloses a dual motor having both a large diameter piston and a small diameter piston for actuation of a single piston rod. The Topinka patent differs from the Hussey patent in that its fluid receiving chambers for the large and small pistons are isolated from each other so as to adapt the motor for various modes of operation. That is, fluid is pumped into separate isolated chambers associated with each piston in order to actuate the piston rod. With the pistons being isolated from each other within the motor housing, an on-off valve may be install within the conduit line supplying fluid to the auxiliary piston and when closed will prevent fluid from being delivered to that piston.
Conventional dual piston designs can cause the primary piston to move before it receives the benefit of the thrust generated from the auxiliary piston. That is, the Hussey patent supplies fluid to the compartment associated with the primary piston and then from that compartment to the compartment associated with the auxiliary piston. The Topinka patent includes isolated chambers wherein it may also supply fluid to the primary piston prior to the auxiliary piston depending upon the efficiency of its fluid delivery system.
The present invention also provides many additional advantages which shall become apparent as described below.